i) Field of the Invention
This invention relates to the collection of gases or ambient air samples. More specifically, the invention relates to a novel flow controller wherein calculated dimensions of a capillary tube are used to introduce a constant flow of sample into any size of pre-evacuated sampling vessel. Any flow rate is theoretically possible and hence, choices of average sampling time can be selected. The time integration property of this new flow controller is a major attribute. It can contribute in extending sampling duration to obtain more relevant data on the mean levels of contaminants. Also, it can be used to collect grab samples and short term sampling can efficiently be controlled. It provides benefits in air quality studies and process control monitoring.
ii) Description of Prior Art
In air quality monitoring, the sampling methodology is a critical step where many requirements must be fulfilled to assure reliability and to optimize precision prior to the laboratory analytical determination. For many target chemicals, regulations are applied in the workplace to mitigate potential health hazards from inhalation. Again, many chemicals are also regulated in the environment considering their local or global effects. For an industrial hygienist or an environmentalist, active methods of ambient air sampling are mostly used such as sampling pumps and sorbent tubes, to characterize the risk and/or to verify compliance.
At present, active sampling devices consist of cumbersome and expensive equipment that can efficiently collect at best only 24-hour integrated samples. With these existing methodologies, the sampling duration is limited by technological considerations for the achievement of low and precise flow rate. Sample size is also reduced when the investment in equipment required for an extensive field study is considered.
In the case of gaseous contaminants such as organic vapors, all present sampling methodologies have an upper integrative time boundary. According to the sampling principles applied in workplace monitoring employing sorbent cartridges, it is still difficult to adequately characterize the nature of mean exposures. Such methods require that enough air be collected at a definite flow rate to assure the validity of laboratory analysis (adequate amount of trapped analytes vs analytical limits of detection).
In order to simplify the sampling procedures and lower the cost of air quality studies, passive techniques have been developed but they lack either precision or versatility. For the measurement of volatile organic compounds in air, a sampling procedure has been developed recently by the U.S. Environmental Protective Agency using a pre-evacuated stainless steel vessel or Summa (Trade Mark) canister as a whole air sampler. With this new sampling procedure integrated subpressurized samples can be collected passively using critical orifices as an inlet mechanical flow controller. This type of flow controller acts as a point restriction for the entry of air or gas sample, and the low flow rates obtained are principally a function of the orifice size. However, the average sampling time cannot exceed a few hours because of physical limitations of the orifice size.
In the development of an overall strategy of sample collection, temporal and spatial considerations are of prime importance. It is necessary to adopt sampling strategies which recognize the inherent statistical nature of assessing air quality. Considering the environmental variability observed in ambient air levels, combined with the chronic or the carcinogenic effects associated with exposure to some chemicals, long-term average concentration provides meaningful information in terms of risk analysis.
Toxicologically, it has been suggested that sampling duration should be adapted to represent the human uptake, distribution and elimination kinetics of these harmful substances so that exposure measurements can be related to the total body burden. For many of the toxic chemicals such as volatile organic chemicals (VOCs), rates of elimination support the use of longer sampling time. Long term integrated sampling can provide a better estimate of the absorbed dose, and correlations between exposure assessment and health effects can be improved.
Statistically, it has been shown that standard deviations calculated for airborne contaminants data collected in one location, or for a class of workers, will be a function of averaging times. The distribution of mean long term integrated measurements has a smaller variance. When comparing workers mean exposure, this observation is very important in testing for compliance. It means that less data would be required to observe statistically significant differences based on legal standards or threshold limit values (TLVs) defined for the workplace. This effect of averaging time on the distribution of air quality measurements also has the same mathematical importance in data handling when environmental levels need to be established to determine global trends.
Based on a legal standpoint, definitions are also in favor of increasing the sampling duration worldwide. For environmental protection, many guidelines are defined as mean levels not to be exceeded over periods of weeks, months or a year. In a workplace, the limits established by the American Conference of Governmental Industrial Hygienists (ACGIH) correspond to normal 8-hour workday and a 40-hour workweek. Under many regulations, arguments support the application of devices which could evaluate airborne contaminants over an extended period of time.
The use of long term monitoring has been justified according to toxicological, statistical and legal criteria. For the benefit of air quality studies, it was shown that actual methodologies should be improved to overcome present drawbacks. Better sampling methods can also find application in solving engineering problems.
In process control, it is sometimes necessary to perform routine monitoring when direct on-line readings systems are not available. Indirect collection of process gases or emissions at the source is then required. For these purposes, the present methodologies have the same sampling time limitations as those found in air quality monitoring. For example, in fluctuating processes such as organic vapors biofilters and scrubbers, it is only possible to estimate the global performance of these gas treatment technologies with a repeated number of short time (hours) samples taken over a significant period (months) of operation. The overall yield is difficult to define. Long term sampling at the inlet and outlet of such technologies can improve the estimation of performance.